1. Field of the Invention
The present invention relates to a power supply apparatus which supplies device drive voltage by boosting power supply voltage.
2. Description of the Related Art
To drive an LCD (liquid crystal display) panel for battery-driven portable equipment, such as cellular phones or PDAs (personal digital assistants), it is necessary to supply a drive voltage, which is a battery voltage of about 3.6V supplied from a lithium ion battery or the like boosted to a high voltage of about 15 V. In battery-driven portable equipment, LED (light-emitting diode) elements are used for a variety of purposes, which include use as a backlight for an LCD (liquid crystal display), as a flash for an attached CCD (charge coupled device) camera or as an illumination with the LED elements flashing in different emission colors. Driving the LED element requires a large current of about 20-100 mA but a low drive voltage of about 4.5 V. Thus, the LCD panel and the LED element require a very different level of the drive voltage.
Since the drive voltage for the LCD panel is considerably higher than the battery voltage, the battery voltage must be boosted in multiple steps by a charge pump circuit or the like. For instance, a power supply circuit for an LCD is disclosed in Reference (1), which receives an input of power supply voltage from a logic unit in a driver IC for driving an LCD panel and boosts the input voltage in multiple steps by a charge pump circuit that uses a large number of capacitors, and thereby generates a high voltage for driving the LCD panel.
Related Art List
    (1) Japanese Patent Application Laid-Open No. 2001-125062.
To boost the battery voltage by a charge pump circuit, a large number of capacitors are necessary according to the number of boosting steps, and therefore it becomes a big problem on component mounting especially when there arises a need to develop portable equipment smaller and thin-shaped. In addition, if boosting circuits are individually provided for the LCD panel and the LED element, which require different drive voltages, for boosting the battery voltage to the respective drive voltages, a mounting area in a power supply IC expands, and therefore it becomes an obstacle in miniaturizing and lightening the portable equipment.